Fair Value Measurements (Details 4) (Recurring Fair Value Measurements, USD $) In Millions, unless otherwise specified																6 Months Ended
	Jun. 30, 2012 Assets (liabilities) measured at fair value	Dec. 31, 2011 Assets (liabilities) measured at fair value	Jun. 30, 2012 Assets (liabilities) measured at fair value Available-for-sale Non-U.S. governments	Jun. 30, 2012 Assets (liabilities) measured at fair value Available-for-sale Corporate debt securities	Jun. 30, 2012 Assets (liabilities) measured at fair value Available-for-sale Collateralized debt obligations	Jun. 30, 2012 Assets (liabilities) measured at fair value Available-for-sale Other debt obligations	Jun. 30, 2012 Assets (liabilities) measured at fair value Trading Fixed maturities	Jun. 30, 2012 Assets (liabilities) measured at fair value Trading Collateralized private investment vehicles Fixed maturities	Jun. 30, 2012 Assets (liabilities) measured at fair value Other investments	Jun. 30, 2012 Assets (liabilities) measured at fair value Separate account assets	Jun. 30, 2012 Assets (liabilities) measured at fair value Investment-type insurance contracts	Jun. 30, 2012 Assets (liabilities) measured at fair value Derivative liabilities	Jun. 30, 2012 Assets (liabilities) measured at fair value Other liabilities	Jun. 30, 2012 Fair value hierarchy Level 3	Dec. 31, 2011 Fair value hierarchy Level 3	Jun. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Non-U.S. governments Discounted cash flow Input	Jun. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Non-U.S. governments Discounted cash flow Weighted average input	Jun. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Discounted cash flow Minimum	Jun. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Discounted cash flow Maximum	Jun. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Corporate debt securities Discounted cash flow Weighted average input	Jun. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Collateralized debt obligations Discounted cash flow Minimum	Jun. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Collateralized debt obligations Discounted cash flow Maximum	Jun. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Collateralized debt obligations Discounted cash flow Weighted average input	Jun. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Other debt obligations Discounted cash flow Input	Jun. 30, 2012 Fair value hierarchy Level 3 Available-for-sale Other debt obligations Discounted cash flow Weighted average input	Jun. 30, 2012 Fair value hierarchy Level 3 Trading Fixed maturities Discounted cash flow Minimum	Jun. 30, 2012 Fair value hierarchy Level 3 Trading Fixed maturities Discounted cash flow Maximum	Jun. 30, 2012 Fair value hierarchy Level 3 Trading Fixed maturities Discounted cash flow Weighted average input	Jun. 30, 2012 Fair value hierarchy Level 3 Other investments Discounted cash flow Input	Jun. 30, 2012 Fair value hierarchy Level 3 Other investments Discounted cash flow Weighted average input	Jun. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, mortgage loans Minimum	Jun. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, mortgage loans Maximum	Jun. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, mortgage loans Weighted average input	Jun. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, real estate Minimum	Jun. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, real estate Maximum	Jun. 30, 2012 Fair value hierarchy Level 3 Separate account assets Discounted cash flow, real estate Weighted average input	Jun. 30, 2012 Fair value hierarchy Level 3 Investment-type insurance contracts Discounted cash flow Minimum	Jun. 30, 2012 Fair value hierarchy Level 3 Investment-type insurance contracts Discounted cash flow Maximum
Unobservable inputs
Assets measured at fair value	$ 129,093.7	$ 124,868.9	$ 13.8	$ 64.7	$ 36.2	$ 5.1	$ 37.8	$ 132.4	$ 89.8	$ 4,123.9				$ 5,107.6	$ 5,044.4
Liabilities measured at fair value	$ (1,831.0)	$ (1,948.4)									$ (179.4)	$ (94.7)	$ (33.6)	$ (360.1)	$ (397.1)
Illiquidity premium (as a percent)																0.50%	0.50%	0.00%	0.10%	0.38%	4.00%	10.00%	7.86%			0.00%	14.00%	3.60%	3.34%	3.34%	0.00%	0.50%
Discount rate (as a percent)																1.60%	1.60%	2.10%	46.00%	12.00%	1.80%	21.80%	14.60%	20.00%	20.00%	2.10%	69.40%	8.50%	4.30%	4.30%	1.00%	12.60%	4.20%	6.50%	10.50%	8.20%
Earnings before interest, taxes, depreciation and amortization multiple																		4.50	6.50	5.50						0.0	6.50	0.50
Probability of default (as a percent)																		0.00%	100.00%	7.50%
Potential loss severity (as a percent)																		0.00%	25.00%	1.90%						0.00%	66.00%	11.10%
Credit spread rate (as a percent)																															0.70%	12.45%	3.52%
Terminal capitalization rate (as a percent)																																		5.50%	9.50%	7.30%
Average market rent growth rate (as a percent)																																		2.30%	5.70%	3.40%
Long duration interest rate (as a percent)																																					2.40%	2.50%
Long-term equity market volatility (as a percent)																																					16.10%	44.50%
Non-performance risk (as a percent)																																					0.90%	2.90%
Lapse rate (as a percent)																																					0.50%	16.00%
Period of swap rate used for calculating Long duration rate																																					20 years	30 years